1. Field of the Invention
This invention generally relates to a photoresist composition and more particularly, to a photoresist composition of the chemical amplification type suitable for forming fine patterns in the manufacture of super-large scale integrated microcircuits.
2. Prior Art
Photoresists are important materials for the fabrication of integrated circuits. These materials are employed for the transfer of a pattern onto a silicon wafer which allows the selective deposition and removal of materials necessary for the construction of the elements which become the circuit. Today, an important goal in integrated circuit fabrication consists of reducing the size of the discrete devices contained in an integrated circuit by as much as possible to incorporate as many devices as possible per unit area. In memory chips this means that a higher density of data can be stored in a smaller area and that ultimately, the size of the electronic device can be further reduced.
To achieve this goal, photoresists with higher sensitivities and resolution capabilities must be developed. Ultimately, the physics of the optics and the irradiating light which are used in imaging are the main limiting factors on the minimum size of the features which can be imaged. Based on these considerations, it has been found that the highest resolution in optical imaging is obtained using the shortest wavelengths of light. This implies that light in the deep ultraviolet region of the spectrum (i.e., 190-300 nm) must be employed to provide optimal resolution capabilities for the next generation of photoresists which are targeted at a feature size of 0.25-0.5 .mu.m. At the present time, no adequate photoresists are commercially available which operate in this region. Similarly, the sensitivity of the photoresist should be less than 15 mJ/cm.sup.2 since typically irradiation sources in the deep UV are relatively weak. To achieve good definition of the imaged patterns and to permit optimal etching and deposition processes, the photoresist should have as high a contrast as possible.
Considerable efforts are currently being expended to develop deep UV sensitive photo-resists which meet the above criteria. The most successful of these efforts have depended on the use of chemically amplified systems. By the term "chemical amplification" it is meant that the photoresist contains a photosensitive element which on irradiation releases a catalytic agent. The catalytic agent then induces a specifically desired image forming chain reaction that results in a photo-multiplication effect which increases the sensitivity. An example of this principle is shown in the equation below. ##STR2## When poly(4-t-butoxycarbonyloxystyrene) is irradiated in the presence of either a diaryliodonium salt or a triarylsulfonium salt, an acid (HX) is produced from the salt which catalytically deblocks the polymer to give isobutylene and poly(4-hydroxystyrene). See H. Ito and C. G. Willson, "Polymers in Electronics", ACS Symp. Ser., 242, 11 (1984). Whereas the initial polymer is insoluble in aqueous basic solution, the deblocked polymer is very soluble in aqueous base. Thus, if films of poly(4-t-butoxycarbonyloxystyrene) are subjected to imagewise exposure through a mask, only those portions of the film which are exposed to light become soluble and are removed when immersed in aqueous base. In this way, a positive reproduction of the mask is obtained.
An additional example of a similar technology is shown in the following equation. See D. A. Conlon, J. V. Crivello, J. L. Lee and M. J. O'Brien, Macromolecules, 22, 509 (1989). ##STR3##
In this case, the poly(4-t-butoxy-styrene) is effectively and catalytically converted to the polyphenol by a small amount of acid generated from the photolysis of the onium salt. Again, the starting polymer is insoluble in aqueous base while the deblocked polymer is readily soluble. This system also may be considered a positive working photoresist.
The aforementioned photoresists based on poly(4-t-butoxycarbonyloxystyrene) and poly-(4-t-butoxystyrene) form the mainstream of chemical amplification type photoresist since they have high sensitivity. These photoresists are used in the deep UV region unlike conventional diazonaphthoquinone-novolak resins which are commonly used as photoresists for the 436 nm and 365 nm portions of the UV spectrum.
A number of positive working photoresist materials of the chemical amplification type heretofore proposed, however, suffer several problems which must be overcome before they can be used in practice. There is a need for improved positive working photoresist materials of the chemical amplification type.